Zirconia-based thermoelectric compositions



IN VEN TORS.

ATTORNEY.

Sheet Char/es l. MQ Vey Newton MQConnaughey Harlan D. Gibson BY Jack V. Sm/'fh C. I. MCVEY ET-AL Z IRCONIA-BASED THERMOELECTRIC COMPOS ITIONS U13-N40 ALIALLSISBH 'IVOIHLOEHB June l0, 1969 Filed Feb. 21, 1968 June 10, 1969 C, MCVEY ETAL 3,449,175

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INVENTORS. Char/es /.MVey Newton MQConnaughey Har/0n D. Gibson BY Jack V. Smith ATTORNEY.

June l0, 1969 C, MCVEY ETAL 3,449,175

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Charles LME Vey Newon MConnaughey Har/0n D. Gibson BY Jack l ATTORNEY.

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TEMPERATURE? C INVENTORS. Charles l. IVI.C Vey Newton MQConnoughey Harlan D. Gibson BY Jack ATTORNEY.

U.S. Cl. 136-239 1 Claim ABSTRACT OF THE DISCLOSURE A thermoelectric element comprising a densied body having electrical leads at opposed portions of said body which comprises a matrix of zirconia and a metal selected from zirconium, titanium, vanadium, and yttrium, a dispersion of said selected metal in said matrix, and a metal oxide selected from yttria, calcia, and a 4f rare earth oxide, when added to said matrix serves to stabilize the zirconia inthe crystalline habit.

BACKGROUND OF THE INVENTION The invention described herein was made in the course of, or under, a contract with the U.S. Atomic Energy Commission.

The present invention relates to thermoelectric compositions capable of direct conversion of heat to electricity and to methods of fabricating said compositions.

More particularly, the invention relates to thermoelectric compositions which can generate electric power directly from a source of heat at temperatures in the range 1000 C. to 2200 C. over a differential in temperature of from 50 C. to as much as 500 C. between hot and cold junctions.

As conducive to a clearer understanding of this invention, reference will be made to a thermoelectric merit factor Z as defined by the relationship Z=S2/pK where S=the Seebeck coecient, p=electrical resistivity, and K=thermal conductivity. The higher the Z factor, the greater the amount of power generation that can be developed from a thermoelectric material for a given energy throughput. The higher the Seebeck coecient, the higher the merit factor when the product of resistivity and conductivity remains constant. Similarly the lower the product of the resistivity and thermal conductivity, the higher the merit factor at a given Seebeck coefficient. It is thus seen that the usefulness of a thermoelectric material is related not only to its Seebeck coefiicient but also to its electrical and thermal conductivity.

It is accordingly an object of this invention to provide a class of thermoelectric compositions which can function effectively to generate electrical power at a temperature of at least 1000 C. and above.

Another object is to provide methods for the fabrication of such compositions.

SUMMARY OF THE INVENTION The thermoelectric compositions of this invention in their useful state consist of an article of manufacture as a shaped body, comprising a matrix of zirconia and a metal selected from zirconium, yttrium titanium, and vanadium, a dispersion of said selected metal in said lmatrix and a metal oxide selected from yttria, calcia, and a 4f rare earth oxide (having an atomic number from 58-7l) which, when added to said matrix serves to stabilize the zirconia in the crystalline habit. The amount of metal dispersed in the zirconia matrix should be such that it does not exceed its solubility limit at the intended op- United States Patent O 3,449,l75 Patented June 10, 1969 erating temperature. Above its solubility limit, the metal will tend to precipitate at grain boundaries and migrate to surfaces eventually forming an electrical short circuit between hot and cold junctions. In this context, the solubility limit for zirconium in zirconia should not exceed 20 weight percent zirconium.

We have found that by Varying the concentration of the selected metal in the dispersed phase within the above described limits, thermoelectric materials can be produced which exhibit high Seebeck coefficients, low electrical resistivity, and low thermal conductivity to yield sufficiently high energy merit factors to signify their effective utility to generate electrical power at temperatures of at least 1000 C. to 2000 C. and above. The zirconia-based materials are therefore useful in power generation devices associated with missiles, nuclear reactors, chemical reactors, and solar imaging devices where such high temperatures are experienced.

The individual thermoelectric generating unit may ltake several standard forms. Each unit is comprised of the basic thermoelectric compositions as a cylinder bonded to concentric outer and inner layers of an electrical lead material which is chemically and physically stable at the intended service conditions. Another modification is to join plates or wafers of the zirconia-based element to electrically conducting contacts in a sandwich-type configuration.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings show various embodiments and examples of the invention and data illustrative of the claimed thermoelectric compositions.

FIG. l shows the effect of electrical resitivity of Y2O3- stabilized zirconia compositions by varying the Y2O3 content at relatively constant zirconium content as a function of temperature.

FIG. 2 shows the effect on electrical resistivity of Y2O3- stabilized zirconia compositions by varying the zirconium content at relatively constant Y2O3 content as a function of temperature.

FIG. 3 shows how the Seebeck coeicient in microvolts/ C. varies with concentration of Zr as a dispersed phase in a matrix of ZrOZ and Y2O3 as a function of temperature.

FIG. 4 shows how the Seebeck coefficient varies as a function of temperature in CaO and Y2O3 stabilized ZrO2 compositions with yttrium titanium, and vanadium as the dispersed metal phase.

FIG. 5 shows the effect of adding a stabilizing oxide as a dispersed phase in a representative thermoelectric composition of this invention as a function of temperature. It will be noted that the heating and cooling curve for a dispersion of zirconium in ZrO2 undergoes considerable hysteresis as its crystalline structure changes from one habit to another. By contrast, the heating and cooling curves are indentical for a composition containing the stabilizing oxide Y2O3.

FIG. 6 shows the optimization parameter S2/ p of several thermoelectric compositions, consisting of dispersions of zirconium metal in a zirconia matrix. For purposes of comparison the optimization parameter of a preferred thermoelectric composition consisting of a dispersion of 12 weight percent Y2O3 and 8 weight percent Zr in a matrix of ZrO2 is shown, all as a function of temperature.

FIG. 7 shows (l) the variation in a dimensionless iigure of merit ZT where Z is the thermoelectric merit factor and T is temperature in C. of a preferred thermoelectric composition (consisting of l2 Y2O3-80 ZrO2-8 Zr) as a function of temperature, and (2) the variation in thermal conductivity of said composition as a function of temperature. This composition is termed the preferred composition because it represents the highest measured 3 ZT figure of merit of those included within the scope of this invention.

FIG. 8 shows the variation of Seebeck coefficient with temperature in two ZrO2-Zr compositions stabilized with 6 percent CaO in one case and 6.5 percent in another.

FIG. 9 shows the optimization parameter S2/ p of the two thermoelectric compositions of FIG. 8 as a function of temperature. For purpose of comparison, the optimization parameter for the preferred thermoelectric composition consisting of l2 Y2O3-80 ZrO2 and 8 Zr is also shown.

FIG. shows the variation in electrical resistivity of three representative ZrOZ-based thermoelectric compositions containing CaO as the cubic habit stabilizer in two cases and equal quantities (6 percent) CaO and CaO2 in the third case. In the rst case the dispersed metallic phase consisted of 9 percent Zr; in the second case equal amounts (4 percent) of Zr and Ti were used; and in the third case the dispersed metal consisted of 8.5 percent Zr.

FIG. l1 shows the electrical resistivity of the three compositions of FIG. 10 as a function of temperature.

FIG. 12 shows the optimization parameter of three Y2O3-ZrO2-Zr thermoelectric compositions as a function of temperature at varying concentrations of zirconium. These curves show the superiority of the preferred composition (containing 8 percent Zr) over compositions containing higher and lower birconium concentrations.

The zirconia-based thermoelectric compositions of this invention can be fabricated by casting or powder metallurgical methods. Casting is effected by mixing the materials in the proportions desired in a tantalum Crucible and fused in an inert atmosphere above 2600 C. In another method process a powdered mixture of zirconia, stabilizing oxide, and metal is blended and then isostatically pressed at about 50,000 p.s.i. in rubber bags. The resultant green strength structures are then sintered at a temperature at least equal to two-thirds of the melting point of the composition. A third method is by hot pressing a powdered mixture of the components in vacuum or inert gas above 1850o C. followed by sintering at a higher temperature. Sintering above 2400 C. for about 2 hours is preferred in order to obtain densities above 95% of theoretical and to obtain a uniform dispersion of the selected metal throughout the zirconia matrix. Sintering below 2200 C. results in heavy grain boundary precipitates which migrate to the surface of the material upon subsequent heating or cooling.

The thermoelectric compositions made in accordance with the foregoing procedures are then machined to cylindrical or wafer size in cases where they are made by casting. Fabrication by powder metallurgy allows the parts to be pressed to the approximate finally desired geometry and dimensions. Electrical connections of low electrical resistance are then applied by gas pressure bonding. Gas pressure bonded contacts with either niobium, tantalum, platinum, or molybdenum applied at a pressure of 10,000 p.s.i. at 1650 C. for one hour at temperature and pressure form excellent junction contacts capable of withstanding repeated thermal cycling.

The following examples represent preferred embodiments of this invention in which zirconium is the preferred metal of the dispersed phase. It should be understood, however, that the same techniques are equally applicable to fabricating and using other thermoelectric compositions falling within the scope of our claim.

EXAMPLE I A series of powdered mixtures containing varying amounts of Y2O3, Z102, and zirconium were homogeneously blended and consolidated into shapes by hot pressing under a pressure of 2500 p.s.i. at 1950" C. Photomicrographs of the resultant densiied structure indicated a matrix consisting of Y2O3, ZrO2, and Zr with a fairly uniform dispersion of Zr in the matrix. Thermal conductivity measurements showed that the inclusion of zirconium metal in the ZrO2-Y2O3 matrix increased the thermal conductivity somewhat over yttria-stabilized zirconia without the metal. On the other hand, there was a corresponding and much larger decrease in the electrical resistivity at high temperature. Thus, for example 8 weight percent zirconium metal in the composition resulted in an increase in thermal conductivity of 15% over yttriastabilized zirconia without Zr, coupled with a corresponding decrease in the electrical resistivity by a factor of 50 at high temperature. This resulted in an improvement in the thermoelectric figure of merit by a factor of 15 at 2000 C. over the Y2O3-ZrO2 combination without the zirconium addition.

The relative effects of yttria and zirconium on the electrical resistivity of the Y2O3-ZrO2-Zr composition are shown in FIGS. 1 and 2. The compositions studied show that the variation in the YZOS-ZrOZ-Zr does not signific'ntly affect the Seebeck coecients but does noticeably change the electrical resistivity requirements. Hence, it is desirable to keep the yttria and any other oxide additive at the minimum concentration required to stabilize the zirconia. The effect of Zr on the Seebeck coefficient and the optimization parameter .S2/p is shown in FIGS. 3, 6, and 12, respectively. FIG. 12 shows that the highest optimization parameter is achieved with 8 weight percent zirconium with lower values shown for higher and lower amounts of zirconium. FIG. 7 shows that a 12 Y2O3-80 Zr2-8 Zr reaches a figure of merit (ZT=TS2/pK) of 0.8 at 2200 C. The relative effects of other metals within the scope of the invention in comparison to Zr and to a Y2O3-ZrO2 composition without a metal additive are shown in FIGS. 3 and 4. The compositions containing titanium and vanadium as additives have lower Seebeck coefficients and higher electrical resistances. Thus, while they constitute improvements over the basic ZrOz-Y2O3 composition because of lower resistivity, they are not as effective as zirconium additions. The compositions containing yttrium metal are closer to zirconium but have a higher electrical resistivity over comparable concentrations. Additions of hafnium resulted in gross deviations in Seebeck coefficients depending on whether the specimen was heated or cooled.

EXAMPLE II A single cell thermoelectric generator was fabricated by ball milling a mixture of l2 Y2O3-80 ZrO2-8 Zr powder to a particle size of less than -325 mesh (44 microns). The mixed powders were isostatically pressed at 70,000 p.s.i. and sintered in argon for 2 hours at 2400D C. The generator consisted of a cylinder (3.6 cm. outer diameter, 2.2 cm. inner diameter, 45 cm. long) slip-fitted on outside and inside diameters with 3.5 mm. thick molybdenum sleeves to serve as electrical contacts. The inside surface temperature, which served as the cold junction, was regulated by varying the ow of an inert gas through a heat exchanger in the bore. The outside surface served as the hot junction. Power testing was conducted by inductively heating the hot junction. At hot and cold junctions of 2200 C. and 1600o C., respectively, the generator produced more than 1.5 watts of electrical power.

The high temperature stability of the sintered 12 Y2O3-80 ZrO2-8 Zr specimen was evaluated by noticing changes in electrical properties. Tests were conducted in which the Seebeck coefhcient and electrical resistivity were simultaneously determined as a function of time with a 400 C. temperature gradient across the specimen at a hot junction temperature of 2000 C. over a period of 28 hours in an argon atmosphere. The results showed that after an initial period (about 8 hours) in which the Seebeck coefficient and electrical resistivity rose slightly, the electrical properties remained constant over the remaining test period. The results were the same for sintered or fused specimens.

EXAMPLE In A thermoelectric device was fabricated as described in Example II from a composition consisting of 93 weight percent Zr02 and 7 weight percent Zr. Its general configuration was cylindrical 2.2 cm. outer diameter, 1.6 cm. inner diameter and 61.5 cm. long. Slip-fitted on the outside and inside diameters were two niobium tubes to act as electrical contacts. The three components making up the thermoelectric module were pressure-bonded rat 700 kilograms/cm.2 -and 1600 C. for one hour completely bond-ed the niobium to the walls of the thermoelectric material and gave a minimum electrical junction resistance. Electrical power was drawn from the thermoelectric module by heating the center of the module and cooling the outside. The module was operated at a hot junction temperature of 2000 C. and a cold junctio-n temperature of 1500 C. for yapproximately 30 hours. A constant current of amperes at 0.1 volt was obtained throughout the test period.

This example shows that the stabilizing oxide additive is not essential from the standpoint of thermoelectric properties. The stabilizing oxide does function, however, to impart dimensional stability to the thermoelectric composition due to phase changes, possible compound formation and in particular to the characteristic reversible m'onoclinic tetragonal phase ch'ange which begins toV occur at about 1000 C. Minor amounts olf the tetragonal phase have been noted even at 1400 C.

The effect olf this ph-ase change on the dimensional stability of the formed thermoelectric composition is shown in FIG. 5 on which heating and cooling curves are plotted Ias 'a function of temperature for thermoelectric compositions with and without yttria. The heating and cooling curve for the specimen without the oxide additive clearly shows a decrease hysteresis effect due to the expansion and contraction of zirconia as it passes from one crystallographic change to another over that for pure zirconia. The advantage of using a phase-stabilizing ooncentration of the selected oxides is thus apparent from FIG. 5. In the case of yttria, the monoclinic phase will disappear when 'as little as 6 or 7 percent is incorporated in the initial powder blend. As increasing amounts, up to 15 percent, are used the zirconia ywill have completely 'been stabilized in the cubic habit. The same general eect will be noted in the other members of the class of stabilizing oxides herein disclosed.

It will be seen that there has been described -a class of zirconia-based thermoelectric compositions capable of generating electrical power at temperatures of the order olf 2000 C. The basic thermoelectric components consist of a matrix of zirconia and a metal selected from zirconium, titanium, vanadium, and yttrium, and a fairly homogeneous dispersion of the selected metal as a dispersed phase. In order to render such an element dimensionably stable against crystalline changes as the element undergoes heating and cooling cycles a metal oxide selected from yttria, calcia, and a 4f rare -earth oxide is added to stabilize the ZrO2 into a single crystalline habit.

In the specification and claim the thermoelectric element is described in terms of its initial components. Thus, while the thermoelectric element is described in terms of zirconia, ZrOz, the nal composition of the fabricated element has probably resulted in conversion of ZrO2 into different chemical forms by combination with the other components of the starting mixture.

All concentrations of metal and oxide are to be read in terms of weight percent of the total composition.

What is claimed is:

1. A thermoelectric element comprising a densied body having electrical leads at opposed portions ott said body, said body formed by a process of intimately mixing zirconia, a metal selected from zirconium, titanium, vana` dium, and yttrium, and a metal oxide selected from yttria, calcia, and a 4f rare earth metal oxide, and densifying the resultant mixture by heating said mixture to at least its sintering temperature in an inert atmosphere and at a pressure suliicient to form a dense coherent body having a matrix of zirconia and said metal oxide in which the concentration of said metal is between about 4 weight percent and its solubility limit in said matrix at the intended service temperature.

References Cited UNITED STATES PATENTS 2,912,477 11/1959 Fischer et al 136-238 3,294,688 12/1966` Precht 136-238 OTHER REFERENCES Brooks, M. H.: (ed.) NP-13266, Thermoelectric Materials, Atomic Energy Comm. Report announced in Nuclear Science Abstracts, Jan. 15, 1964.

JOHN H. MACK, Primary Examiner. A. BEKELMAN, Assistant Examiner-- U.S. Cl. X.R. 

